The present invention relates to a force-measuring apparatus, particularly a weighing cell, with a stationary part and a load receiver that serves to receive the force to be measured. The load receiver is coupled to the stationary part so that, in response to the force, the load receiver is movable in relation to the stationary part. The force-measuring apparatus includes a lever that is connected to the load receiver through a first connector element and to the stationary part through a second connector element. The first and second connector elements are located at a distance from each other in the lengthwise direction of the lever and are part of an arrangement that allows the lever ratio to be changed. The second connector element forms the fulcrum axis for the rotation of the lever as the latter transmits the displacement of the load receiver to a lever output area located at a distance from the first and second connector elements in the lengthwise direction of the lever.
Force-measuring apparatus of this kind are described, e.g., in the Swiss patent CH 591 684 and incorporate the concept that, in many cases, a base model type of a weighing cell is followed by one or more derivative models that differ from the base model in regard to the weighing range or the measuring resolution. According to the aforementioned patent, the number of components required to manufacture weighing cells of this kind can be reduced by changing the arm ratio of the lever that serves to transmit the displacement motion of the load receiver to a compensation coil immersed in the gap of a permanent magnet assembly that is attached to the stationary part. As proposed in that patent, this can be accomplished by connecting a one-armed lever to the stationary part through two tensile flexures so that the lever is rotatable about a fulcrum axis formed by the tensile flexures in an arrangement where the lever arm length between the fulcrum axis and an attachment area for connecting the lever to the load receiver can be changed by means of spacer blocks. The spacer blocks can be arranged on the one hand between the lever and the tensile flexures and on the other hand between the stationary part and the tensile flexures. It is further proposed in the aforementioned patent that appropriate spacer blocks be inserted also at the respective attachment areas of the load receiver and/or the lever where a tensile flexible coupling element is attached that connects the load receiver with the lever. A similar weighing cell, in which the concept of changing lever ratios with spacer blocks is applied to a two-armed lever, is described in the Swiss patent 591 685.
The weighing range of these known load cells can be selected from at least two possible values by changing the position of the spacer blocks and thereby changing the lever ratio between the displacement of the load receiver and the displacement of the compensation coil.
However, with the known weighing cells, it has been found that changing the positions of the spacer blocks in order to change the weighing range is generally a time-consuming process and can also be harmful to the measuring accuracy.
In view of the aforementioned problems inherent in state-of-the-art devices, it is therefore the object of the present invention to provide a force-measuring apparatus that allows the measuring range to be changed quickly and reliably.
According to the invention, this goal is accomplished through a design advancement of the known force-measuring apparatus. The essential characteristic feature of the inventive design is that the lever has at least three attachment areas for fastening one of the first and second connector elements, and that the attachment areas are set apart at a distance from each other in the lengthwise direction of the lever.
The invention is based on the observation that the problems with the state-of-the-art solutions mentioned above are essentially caused by the way the spacer blocks are handled for changing the measuring range. Namely, in a first step, the tensile and pivotal flexures have to be disconnected from the lever, the load receiver, and the stationary part. Next, the spacer blocks are removed from their original positions and arranged in their new positions. In a final step, in which the tensile and pivotal flexures are reattached, the spacer blocks have to be kept precisely fixed in their new positions. This complicates the handling of the tensile and pivotal flexures in the assembly process because of the additional time required to change the measuring range. In addition, it is possible for the spacer blocks to be dislodged from their intended new positions, which is detrimental to the measuring accuracy.
The force-measuring apparatus with the inventive design advancements is free of the problems associated with the handling of the spacer blocks, because the lever itself has at least three attachment areas for fastening a connector element, which are set apart at a distance from each other in the lengthwise direction of the lever. In each measuring range, only two of the attachment areas are used, so that the measuring range can be set or changed by simply selecting or changing the attachment areas used for fastening the connector elements without the need for the time-consuming and error-prone process of handling spacer blocks. As a result, the time required to change measuring ranges is shortened, while at the same time the functional reliability of the force-measuring apparatus is increased.
In addition, from a manufacturing point of view, the force-measuring apparatus according to the invention has the advantage that one and the same lever component can be used to manufacture balances with different weighing ranges by fastening the connector elements to different respective attachment areas. This manufacturing advantage is also particularly useful even in the case where no change in measuring range has to be made during the actual operation of the force-measuring apparatus.
The process of changing measuring ranges of a force-measuring apparatus in accordance with the invention can be further simplified through an arrangement where at least two attachment areas are set apart at a distance from each other in the transverse direction of the lever, because this allows a connector element to be fastened to one of the attachment areas without spatial interference by a connector element already fastened to the other attachment area.
The fulcrum axis formed by the second connector element can be defined with particular accuracy while, at the same time, the lever is stabilized against tilting in relation to its longitudinal axis, if at least one of the attachment areas for the second connector element has at least two attachment area segments at a distance from each other along the fulcrum axis. In this arrangement, the second connector element can be, for example, a pair of tensile pivotal flexures.
The process of changing measuring ranges of a force-measuring apparatus in accordance with the invention can be further simplified through an arrangement where at least one attachment area comprises an attachment surface approximately perpendicular to the longitudinal axis of the lever and located at a frontal end surface of the lever at the opposite end from the lever output area, because this arrangement allows a connector element to be fastened to the attachment surface without spatial interference by a measuring transducer, e.g., an electromagnetic force-compensation system, which is normally arranged at the output area of the lever. In particular, the inventive concept includes the possibility that the aforementioned attachment surface consists of a plurality of attachment surface segments set apart at a distance from each other in the direction of the fulcrum axis. The inventive concept further includes an embodiment where the attachment surface for the first connector element is arranged parallel to the fulcrum axis between two attachment area segments for the second connector element.
The process of changing measuring ranges of a force-measuring apparatus in accordance with the invention can be further simplified through an arrangement where the load receiver and the stationary part in total have at least three coupling areas for fastening one of the connector elements. In the lengthwise direction of the lever, the coupling areas are spaced apart from each other at analogous positions to the attachment areas on the lever. With this arrangement of the coupling areas on the load receiver and the stationary part, it is possible to change measuring ranges without changing the position of the lever in relation to the load receiver and the stationary part, because for each of the attachment areas of the lever there is a corresponding coupling area available on the load receiver or the stationary part. Thus, the measuring range can be set without changing the position of the lever simply by selecting the attachment areas and their associated coupling areas to which the connector elements are to be fastened.
For the same reasons as given above for the preferred arrangement of the attachment areas, it is particularly advantageous if at least two coupling areas are set apart from each other in a direction parallel to the fulcrum axis of the lever and if at least one coupling area of the stationary part has at least two coupling area segments arranged side-by-side at a distance from each other along the fulcrum axis. This also allows the coupling areas to have coupling surfaces perpendicular to the lengthwise direction of the lever and facing in the opposite direction from the output area of the lever. This further makes it possible to use simple connector elements comprising one or more pivotal flexures, provided that the coupling areas are coplanar with the respective associated attachment areas.
Particularly with the immediately preceding embodiment of the invention, it is possible to set different measuring ranges with only two connector elements, each of which can comprise one or more pivotal flexures, if the coupling areas and their associated attachment areas are located at appropriately corresponding positions and designed so that one connector element can be used in more than one position.
The process of changing measuring ranges of a force-measuring apparatus in accordance with the invention can be further simplified through an arrangement where at least one of the attachment areas and/or coupling areas is designed for fastening a connector element with one or more screws, particularly by providing at least one threaded hole.
In a particularly advantageous configuration of the inventive apparatus, the attachment and/or coupling areas are about mirror-symmetric in relation to a central plane that is perpendicular to the fulcrum axis, because a particularly high degree of measuring accuracy is achievable with this arrangement. As a part of this configuration, it is practical if at least one attachment for the first connector element and/or a coupling area of the load receiver is arranged parallel to the fulcrum axis between two attachment area segments for the second connector element and/or two coupling area segments of the stationary part.
A force-measuring apparatus according to the invention can be given a particularly compact design envelope if, in the lengthwise direction of the lever, the load receiver is arranged between the attachment areas and the lever output area, preferably in an opening of the lever. To provide a coupling area for a first connector element so that the coupling area is coplanar with the associated attachment area at the frontal end surface of the lever, the load receiver is preferably equipped with a cantilevered portion carrying the coupling area for the first connector element.
To achieve the most compact configuration possible in a force-measuring apparatus according to the invention, it has proven to be especially advantageous if the stationary part comprises two lateral parts extending in planes essentially perpendicular to the fulcrum axis and if the lever is arranged essentially between the two lateral parts. In addition to a particularly compact configuration, this arrangement also protects the lever from being dislodged accidentally by a force from the side. In the preceding embodiment of the invention, the mirror-symmetric arrangement of the coupling area segments of the stationary part can be realized in a particularly simple manner if each of the lateral parts comprises at least one segment of a coupling area. To avoid spatial interference between the coupling elements and the measuring transducer that is normally arranged at the lever output area, it is practical for the coupling area segments to be arranged at the ends of the lateral parts on the opposite side from the lever output area. With this arrangement it can be achieved that, in a load-free condition of the system, the connection between the lever and the stationary part is subjected to only tensile forces, if the lever has at least one projection extending about parallel to the fulcrum axis in the direction towards one of the lateral parts with at least one attachment area segment and if the attachment area segment of the projection lies in the same plane, perpendicular to the lengthwise direction of the lever, as the associated coupling area segment of the respective lateral part.
As has already been explained, a practical arrangement for an attachment area of the lever for a first connector element is between the ends of the lateral parts. With this arrangement it can be achieved that, in a load-free condition of the system, the connection between the lever and the load receiver is subjected to only tensile forces, if a coupling area of the load receiver lies in the same plane, perpendicular to the lengthwise direction of the lever, as the associated attachment area of the lever.
To avoid measuring errors that could occur as a result of applying a load eccentrically to a weighing pan or load plate attached to the load receiver, a particularly preferred arrangement is for the load receiver to be guided in a mode of parallel motion in relation to the stationary part by two parallel guide members that are stiff in their longitudinal direction but allow flexible movement in the transverse direction. Each of the two parallel guide members is attached at one end to the load receiver and at the opposite end to the stationary part. This arrangement allows a particularly compact design of the inventive apparatus, if the lengthwise direction of the lever is approximately parallel to the guide members and the lever lies essentially between the parallel planes of the guide members. To ensure the absence of measuring errors caused by an eccentric load, it is necessary for the guide members to be exactly parallel to each other. As a practical means of ensuring the exact parallel alignment of the guide members, the inventive apparatus is equipped with an adjusting device by which the position of at least one attachment terminal where one of the guide members is mounted to the stationary part can be adjusted in the direction transverse to the plane of the guide member. In a particularly simple embodiment of this concept, the attachment terminal is connected to an arm that is about parallel to the guide members and is pivotally connected to the stationary part so that it can swivel up or down in relation to the latter. The position of the arm is secured or changed by a position-adjuster element holding the arm at an adjustment area.
In an advantageous embodiment of the invention, the distance from the adjustment area of the arm to the attachment terminal for the guide member is at least exactly as long as the distance from the attachment terminal to the pivotal connection of the arm. Thus, when the position of the arm is being changed by means of the position-adjuster element, the displacement of the adjustment area is leveraged down by a factor of more than two at the attachment terminal so that a precise position adjustment of the attachment terminal is possible even with only a simple adjustment screw as a position-adjuster element.
It is practical for the arm that makes up part of the adjustment device to be coupled to the stationary part through a pivotal flexure and, preferably, to be formed as an integral, monolithically connected portion of the stationary part. A particularly compact design version of the inventive force-measuring apparatus is made possible if the arm extends lengthwise from its pivotal connection in the direction towards the load-receiver ends of the guide members. An arm of this kind can be realized, e.g., with a slit in the respective lateral part. If the guide members are attached to both of the lateral parts, it has proven to be particularly advantageous if each of the lateral parts comprises an arm with an attachment terminal, each of the arms being adjustable by means of an adjuster element so that the attachment terminals of the two arms can be adjusted independently of each other.
In the interest of a particularly compact configuration, it is preferred if at least one arm of the adjustment device extends essentially in the space between the planes of the guide members in a parallel plane to the guide members.
An undesired torsional deformation of the arm in the area of the attachment terminal can be avoided without increasing the weight of the apparatus, if a section of the arm comprising the attachment terminal is thicker in the direction transverse to the planes of the guide members than a section of the arm comprising the adjustment area.
An undesirable sideways displacement of the arm in the direction parallel to its pivotal axis can be prevented, if the inventive force-measuring apparatus includes a stabilizer element that is connected at one end to the arm and at the other end to the body of the stationary part, so that it stiffens the arm against moving sideways in parallel with the planes of the guide members while allowing movement perpendicular to the planes of the guide members, preferably by being elastically flexible.
From the point of view of optimizing the construction, it has proven to be particularly advantageous if the stabilizer element is formed in a monolithic unit together with the arm and/or the body of the stationary part as an essentially S-shaped connector portion as seen in a plane that is perpendicular to the pivotal axis of the arm. An additional increase in the torsional rigidity of the arm is achieved if the width of the connector portion in the direction parallel to the pivotal axis of the arm is significantly greater than the thickness in the perpendicular direction.
To achieve the most compact configuration possible for the inventive force-measuring apparatus, it has proven to be particularly advantageous if, in relation to the lengthwise direction of the arm, the stabilizer element is arranged essentially at a location between the adjustment area and the attachment area. A further improvement in utilizing the remaining free space is achieved, if the stabilizer element is arranged essentially between the guide members, as seen in a sectional plane that is perpendicular to the pivotal axis.
In the following, the invention is explained on the basis of the drawing, which is also being explicitly referred to with regard to all details essential to the invention that are not closely portrayed in the description.